Active Microwave Imaging II: 3-D System Prototype and Image Reconstruction From Experimental Data

A 3-D microwave imaging system prototype and an inverse scattering algorithm are developed to demonstrate the feasibility of 3-D microwave imaging for medical applications such as breast cancer detection with measured data. In this experimental prototype, the transmitting and receiving antennas are placed in a rectangular tub containing a fluid. The microwave scattering data are acquired by mechanically scanning a single transmit antenna and a single receive antenna, thus avoiding the mutual coupling that occurs when an array is used. Careful design and construction of the system has yielded accurate measurements of scattered fields so that even the weak scattered signals at S₂₁ = -90 dB (or 30 dB below the background fields) can be measured accurately. Measurements are performed in the frequency domain at several discrete frequencies. The collected 3-D experimental data in fluid are processed by a 3-D nonlinear inverse scattering algorithm to unravel the complicated multiple scattering effects and produce high-resolution 3-D digital images of the dielectric constant and conductivity of the imaging domain. Dielectric objects as small as 5 mm in size have been imaged effectively at 1.74 GHz.

I. Introduction

It is of practical significance to detect, locate, characterize, and image tumors in healthy tissue of the breast. Over the last two decades, intensive investigations have been conducted for early breast cancer detection using microwaves (e.g., [1]–[23]). These studies include confocal microwave imaging [6], [7], [12]–[14], 2-D microwave topographic imaging [8], [9], [11], and 3-D active microwave tomographic imaging [15]–[17]. Microwave imaging has been proposed for breast detection [8], [9], [6], [7] because of its potentially high specificity for breast cancer diagnosis due to the high contrast in electrical properties between normal and malignant human breast tissues. the electrical properties of normal and malignant human breast tissues have been a subject of great interest over the last two decades. It has been reported that a significant contrast in electrical properties at microwave frequencies exists between normal and malignant human breast tissues [3]. This high contrast is due to significantly different sodium concentrations, fluid contents, and electrochemical properties. A recent research on the variability of normal breast tissue properties [21] reveals that the breast can, in fact, be quite heterogeneous, and this may pose a significant challenge for microwave breast imaging, although actual clinical tests in [22] and [23] have demonstrated the significant potential and new advances of microwave breast imaging from previous simulations and phantom studies.